1. Field of the Invention
This invention relates to balun circuits for coupling between balanced and unbalanced lines or devices in an electronic system. More particularly, this invention relates to a miniaturized multi-layer balun circuit for use in mobile communication devices such as portable telephones and cordless telephones.
2. Description of Related Art
Typically, a balun is used to couple a two-line balanced circuit, such as a cellular telephone circuit, to a single-line (unbalanced) circuit, such as an antenna circuit. The following references provide background information relating to baluns and are incorporated by reference herein in their entireties:
[1] U.S. Pat. No. 4,994,755 to Titus et al., entitled xe2x80x9cActive Balun,xe2x80x9d Feb. 19, 1991;
[2] U.S. Pat. No. 5,039,891 to Wen et al., entitled xe2x80x9cPlanar Broadband FET Balun,xe2x80x9d Aug. 13, 1991;
[3] U.S. Pat. No. 5,574,411 to Apel et al., entitled xe2x80x9cLumped Parameter Balun,xe2x80x9d Nov. 12, 1996;
[4] S. A. Maas, xe2x80x9cMicrowave Mixersxe2x80x9d, Artech House, pp 244-255;
[5] U.S. Pat. No. 5,455,545 to Garcia, entitled xe2x80x9cCompact Low-loss Microwave Balun,xe2x80x9d Oct. 3, 1995;
[6] U.S. Pat. No. 4,725,792 to Lampe, Jr., entitled xe2x80x9cWideband Balun Realized By Equal-Power Divider and Short Circuit Stubs,xe2x80x9d Feb. 16, 1988;
[7] U.S. Pat. No. 4,460,877 to Sterns, entitled xe2x80x9cBroad-Band Printed-Circuit Balun Employing Coupled Strip All Pass Filter,xe2x80x9d Jul. 17, 1984;
[8] U.S. Pat. No. 5,497,137 to Fujiki, entitled xe2x80x9cChip Type Transformer,xe2x80x9d Mar. 5, 1994;
[9] U.S. Pat. No. 5,025,232 to Pavio, entitled xe2x80x9cMonolithic Multilayer Planar Transmission Line,xe2x80x9d Jan. 18, 1991;
[10] U.S. Pat. No. 4,847,626 to Kahler et al., entitled xe2x80x9cMicrostrip Balun-Antenna,xe2x80x9d Jul. 11, 1989; and
[11] U.S. Pat. No. 4,755,775 to Marczewski et al., entitled xe2x80x9cMicrowave Balun for Mixers and Modulators,xe2x80x9d Jul. 5, 1988.
The term xe2x80x9cbalunxe2x80x9d is a contraction of balanced to unbalanced. A balun is a RF balancing network or electric circuit for coupling an unbalanced line or device and a balanced line or device for the purpose of transforming from balanced to unbalanced or from unbalanced to balanced operation, with minimum transmission losses and high impedance transformation ratio. A balun is normally used between equipment and transmission lines or between transmission lines and antennas. A balun can be used with an unbalanced input and a balanced output or, in the reverse situation, a balanced source and an unbalanced load. Baluns can be used to interface an unbalanced input with a balanced transmission line by dividing the signal received at its unbalanced terminal equally to two balanced terminals and by providing the signal at one balanced terminal with a reference phase and the signal at the other balanced terminal with a phase equal to the reference phase plus or minus 180xc2x0. Baluns can be used to interface a balanced or differential input from a balanced pair of two unbalanced transmission lines providing output signals which are 180xc2x0 out-of-phase (odd-mode excitation) and an unbalanced load driven by a single-ended input signal. The balun combines the signals of the balanced input and provides the combined signal at an another port.
A balanced line has two very closely spaced current paths (usually wires), each displaying an equal impedance with respect to ground. At all physical points along the line, the currents in the two paths are equal in magnitude and opposite in direction. Because the two paths are very closely spaced in relation to the wavelength of the signal they carry, their electromagnetic fields cancel each other everywhere in space except in the immediate vicinity of the line. The balanced structure is usually needed in devices such as balanced mixers, modulators, attenuators, switches and differential amplifiers, since balanced circuits can provide better circuit-to-circuit isolation, dynamic range, and noise and spurious signal cancellation. A balanced load is defined as a circuit whose behavior is unaffected by reversing the polarity of the power delivered thereto. A balanced load presents the same impedance with respect to ground, at both ends or terminals. A balanced load is required at the end of a balanced transmission line to ensure that the currents in the line will be equal and opposite.
Depending on the implementation, baluns can be divided into two groups: active and passive. Active baluns are described in references [1] and [2] and are constructed by using several transistors (so-called active devices). Although active baluns are very small, they are not generally preferred for the following reasons. First, due to the employment of active devices, noise will be introduced into the system. Also, active devices tend inherently to waste power; this makes them quite disadvantageous in radio telephone systems. Additionally, the low-cost fabrication of active baluns is limited to semiconductor manufacture. Conversely, passive baluns are quite popular. Passive baluns can be categorized into lumped-type baluns, coil-type baluns, and distributed-type baluns.
Lumped-element-type baluns are described in references [3] and [4]. Lumped-element baluns employ discrete components that are electrically connected, such as lumped element capacitors and lumped element inductors. Advantages of lumped-element-type baluns include small size and suitability for low frequency range usage. On the other hand, the performance of lumped-element-type baluns is not good in high frequency ranges (several Ghz), because the lumped elements are very lossy and difficult to control. Also, the operational bandwidth of lumped-element-type baluns is small ( less than 10%, typically).
Coil-type baluns (trifilar transformers) are very popular in applications in the UHF band or lower frequency range. Shortcomings of the trifilar transformer include unacceptable lossiness in the frequency range higher than the UHF band, and barriers to miniaturization beyond a certain size.
There are many kinds of distributed-type baluns. The first type is the 180xc2x0 hybrid device described in references [4] and [5]. They are constructed by several sections of quarter-wavelength transmission lines and a section of half-wavelength transmission line. The drawbacks of the 180xc2x0 hybrid device are size, difficulty in achieving a high impedance transformation ratio, and limitation to a balanced pair of unbalanced outputs. A second type is the combination of a power divider and a 180xc2x0 phase shifter as described in references [6] and [7]. Since the 180xc2x0 phase shift is achieved by a half-wavelength length difference, the size is still too large. The third type is the well-known Marchand-type balun as described in references [8]-[11]. This type of balun has very wide bandwidth (multi-octave). Further, both the phase balance and the amplitude balance are excellent. Moreover, it can be applied not only in a balanced port (load) but also in a balanced pair of unbalanced transmission lines.
A Marchand-type balun is illustrated in FIG. 1, and its equivalent circuit is shown in FIG. 2. In FIG. 1, balun 10 is constructed by a substrate 12 having formed on one surface a transmission line structure defined by a top conductive strip 14 and interlevel conductive strips 16 and 17, separated by an interlevel dielectric layer 13. A ground plane electrode 18 is formed on the opposing planar surface of dielectric substrate 12. Top conductor 14 includes a relatively narrow section 14-1 and a relatively wide section 14-2. Interlevel conductor 16 underlies the top section 14-1, and interlevel conductor 17 underlies the top section 14-2. Top conductor 14 is continuous in length, while interlevel conductors 16 and 17 are separated by a central balance point gap G that centers on the transition between the 14-1 and 14-2 sections of top conductor 14. Interlevel conductors 16 and 17 are electrically isolated from one another and are connected through via-holes 15 to ground plane electrode 18. A load may be coupled across the balance point gap G via a pair of microstrip transmission line strips 16-1 and 17-1 extending from the balance point ends of respective interlevel conductor strips 16 and 17. Strips 16-1 and 17-1 thus constitute the balanced port BP. Input terminals may be connected across one of interlevel conductors 16 and 17 and the corresponding sections 14-1 and 14-2 of top conductor 14, to provide the unbalanced port UBP. The length of each of the interlevel conductor sections 16 and 17 is xc2xc xcex. The width of the top conductor section connected to the unbalanced port UBP controls the impedance transformation ratio. This section is equivalent to a xc2xc xcex impedance transformer. A drawback to this configuration is that the size is xc2xdxcex. In RF applications, this size is still too large. In reference [8], the size is reduced by a zigzag and spiral arrangement. Under such arrangement, the modified Marchand-type balun can be chip-sized. However, the discontinuities of the spiral and zigzag arrangement will introduce some losses.
One object of the present invention to reduce the size of the conventional Marchand-type balun circuit.
It is another object of the present invention to minimize the number of spiral turns for a fixed size to decrease losses resulting from discontinuities.
According to a first embodiment of the invention, there is provided a balun circuit comprising a dielectric substrate having substantially planar opposing surfaces; a groundplane conductor layer disposed on a first one of the opposing surfaces of the dielectric substrate; an interlayer conductor layer disposed on a second one of the opposing surfaces of the dielectric substrate and comprising first and second conducting strips electrically isolated from each other and having a balance point gap between first and second ends thereof, wherein balanced port terminals are provided on respective sides of the balance point gap, said first and second conducting strips having second ends that are short-circuited to the groundplane conductor layer; an interlayer dielectric layer having substantially planar opposing surfaces, with a first one of the opposing surfaces of the interlayer dielectric layer being disposed over the interlayer conductor layer; and a top conductor layer disposed over a second one of the opposing surfaces of the interlayer dielectric layer and comprising a third conducting strip overlying the first and second conducting strips, one end of the third conducting strip providing an unbalanced port terminal and another end of the third conducting strip being open-circuited, wherein the third conducting strip comprises a first set of series-connected line sections having diverse impedances and a second set of series-connected line sections having diverse impedances which are a mirror opposite of the diverse impedances of the first set of line sections relative to a center plane of the balun circuit passing through the balance point gap and being orthogonal to the opposing surfaces of the dielectric substrate, and the first conducting strip has an impedance which is a mirror opposite of an impedance of the second conducting strip relative to the center plane of the balun circuit, whereby phase and amplitude balance at the balance point gap is achieved by the mirror opposite relationship of the impedances of the first and second set of line sections and the mirror opposite relationship of the impedances of the first and second conducting strips.
Now there will be described various detailed alternative configurations of the first embodiment.
In one configuration of the balun circuit according to the first embodiment, the first set of line sections having diverse impedances comprises a first segment and a second segment connected to one another and having different widths to provide a stepped impedance junction, the first segment being closer to the center plane of the balun circuit and being narrower than the second segment, and the second set of line sections having diverse impedances comprises a third segment and a fourth segment connected to one another and having different widths to provide a stepped impedance junction, the third segment being closer to the center plane and being narrower than the fourth segment.
In another configuration of the balun circuit according to the first embodiment, the first set of line sections having diverse impedances comprises a first segment and a second segment connected to one another and having different widths to provide a stepped impedance junction, the first segment being closer to the center plane of the balun circuit and being wider than the second segment, and the second set of line sections having diverse impedances comprises a third segment and a fourth segment connected to one another and having different widths to provide a stepped impedance junction, the third segment being closer to the center plane and being wider than the fourth segment.
In another configuration of the balun circuit according to the first embodiment, the first conducting strip comprises a third set of series-connected line sections having diverse impedances, the second conducting strip comprises a fourth set of series-connected line sections having diverse impedances which are a mirror opposite of the diverse impedances of the third set of series-connected line sections relative to the center plane of the balun circuit, and the third set of line sections having diverse impedances comprises a fifth segment and a sixth segment connected to one another and having different widths to provide a stepped impedance junction, the fifth segment being closer to the center plane of the balun circuit and being narrower than the sixth segment, and the fourth set of line sections having diverse impedances comprises a seventh segment and an eighth segment connected to one another and having different widths to provide a stepped impedance junction, the seventh segment being closer to the center plane of the balun circuit and being narrower than the eighth segment, whereby impedance characteristics of the segments providing the stepped impedance junctions are set to achieve impedance matching of the unbalanced port and the balanced port.
In yet another configuration of the balun circuit according to the first embodiment said first conducting strip comprises a third set of series-connected line sections having diverse impedances, the second conducting strip comprises a fourth set of series-connected line sections having diverse impedances which are a mirror opposite of the diverse impedances of the third set of series-connected line sections relative to the center plane of the balun circuit, and the third set of line sections having diverse impedances comprises a fifth segment and a sixth segment connected to one another and having different widths to provide a stepped impedance junction, the fifth segment being closer to the center plane and being wider than the sixth segment, and the fourth set of line sections having diverse impedances comprises a seventh segment and an eighth segment connected to one another and having different widths to provide a stepped impedance junction, the seventh segment being closer to the center plane of the balun circuit and being wider than the eighth segment, whereby impedance characteristics of the segments providing the stepped impedance junctions are set to achieve impedance matching of the unbalanced port and the balanced port.
In another configuration of the balun circuit according to the first embodiment, the top conductor layer further comprises a fourth conducting strip interconnecting the first set of line sections and the second set of line sections, the fourth conducting strip minimizing degradation of the amplitude balance at the balance point gap.
In another configuration of the balun circuit according to the first embodiment, there is additionally provided a chip capacitor having one end connected to the third conducting strip at the center plane of the balun circuit and another end connected to the groundplane conductor layer.
In the balun circuit according to the first embodiment, the first, second and third conducting strips can have one of a straight configuration, a spiral configuration and a zigzag configuration.
According to a second embodiment of the present invention, there is provided a balun circuit comprising a first dielectric substrate having substantially planar opposing surfaces; a first groundplane conductor layer disposed on a first one of the opposing surfaces of the first dielectric substrate; an interlayer conductor layer disposed on a second one of the opposing surfaces of the first dielectric substrate and comprising first and second conducting strips electrically isolated from each other and having a balance point gap between first and second ends thereof, wherein balanced port terminals are provided on respective sides of the balance point gap, said first and second conducting strips having second ends that are short-circuited to the groundplane conductor layer; an interlayer dielectric layer having substantially planar opposing surfaces, with a first one of the opposing surfaces of the interlayer dielectric layer being disposed over the interlayer conductor layer; a top conductor layer disposed over a second one of the opposing surfaces of the interlayer dielectric layer and comprising a third conducting strip overlying the first and second conducting strips, one end of the third conducting strip providing an unbalanced port terminal and another end of the third conducting strip being open-circuited; a second dielectric layer having substantially planar opposing surfaces, with a first one of the opposing surfaces of the second dielectric layer being disposed over the top conductor layer; and a second groundplane conductor layer disposed on a second one of the opposing surfaces of the second dielectric layer, wherein the third conducting strip comprises a first set of series-connected line sections having diverse impedances and a second set of series-connected line sections having diverse impedances which are a mirror opposite of the diverse impedances of the first set of line sections relative to a center plane of the balun circuit passing through the balance point gap and being orthogonal to the opposing surfaces of the dielectric substrate, and the first conducting strip has an impedance which is a mirror opposite of an impedance of the second conducting strip relative to the center plane, whereby phase and amplitude balance at the balance point gap is achieved by the mirror opposite relationship of said impedances of said first and second set of line sections and said mirror opposite relationship of the impedances of the first and second conducting strips. In the second embodiment, there exist various detailed alternative configurations like those discussed above with regard to the first embodiment.
According to a third embodiment of the present invention, there is provided a balun circuit comprising a dielectric substrate having substantially planar opposing surfaces; a groundplane conductor layer disposed on a first one of the opposing surfaces of the dielectric substrate; a first conductor layer disposed on a second one of the opposing surfaces of the dielectric substrate and comprising first and second conducting strips electrically isolated from each other and having a balance point gap between first ends thereof, wherein balanced port terminals are provided on respective sides of the balance point gap, the first and second conducting strips having second ends that are short-circuited to the groundplane conductor layer; a second conductor layer disposed on the second one of the opposing surfaces and comprising a third conducting strip spaced apart from and substantially parallel to the first and second conducting strips, one end of the third conducting strip providing an unbalanced port terminal and another end of the third conducting strip being open-circuited; a third conductor layer disposed on the second one of the opposing surfaces and comprising a fourth conducting strip spaced apart from and substantially parallel to the first, second and third conducting strips, the fourth conducting strip being connected to the groundplane conductor layer; and a plurality of bond wires interconnecting the third conductor layer with the first and second conducting strips, wherein the third conducting strip comprises a first set of series-connected line sections having diverse impedances and a second set of series-connected line sections having diverse impedances which are a mirror opposite of the diverse impedances of the first set of line sections relative to a center plane of the balun circuit passing through the balance point gap and being orthogonal to the opposing surfaces of the dielectric substrate, and the first conducting strip has an impedance which is a mirror opposite of an impedance of the second conducting strip relative to the center plane of the balun circuit, whereby phase and amplitude balance at the balance point gap is achieved by the mirror opposite relationship of the impedances of the first and second set of line sections and the mirror opposite relationship of the impedances of the first and second conducting strips.
In the third embodiment, there exist various detailed alternative configurations like those discussed above with regard to the first embodiment.